Hydrophobic silica fine powder is generally produced by first forming a hydrophilic silica fine powder and treating the powder with silicon compounds having hydrophobic groups. In the most common known procedure, tetrachlorosilane in a vapor phase is subjected to high-temperature hydrolysis by an oxyhydrogen flame to form a hydrophilic silica fine powder commonly known as fumed silica. Then, hexamethyldisilazane (HMDS) or dimethyl-dichlorosilane and water are added to the fumed silica. Condensation reaction of silanol-bearing hydrophobic groups resulting from hydrolysis with silanol on the silica surface is effected at 100 to 300° C. to form siloxane bonds, whereby the silica surface is covered with hydrophobic groups. In this way, silica fine powder exhibiting hydrophobic properties is obtained.
Silica fine powder having a large specific surface area indicative of fine particles tends to partially bind through siloxane bonds to take network and dumbbell structures even when synthesized at temperatures below the melting point of silica: 1,423° C. Additionally, water which is added for hydrolysis of the treating agent used for hydrophobization of silica fine powder causes the silica fine powder to agglomerate, interfering with the dispersion of the hydrophibizing agent such as trimethylsilanol or dimethyldisilanol. In subsequent heat treatment for hydrophobization in the presence of by-products including ammonia, acid (such as hydrogen chloride) and alkali, agglomerated portions mediated with water give rise to binding of silica. This results in a bulky powder in which network and dumbbell portions are complexly entangled to define voids that remain empty and which powder is difficult to handle during storage and transportation. When the powder is dispersed in a liquid, the particles behave like coarse particles and settle down due to insufficient hydrophobization and a complex partially bound structure, resulting in poor dispersion. When such silica fine powder is added to a silicone composition and kneaded therewith, strong shear forces cause the silica bonds to be cleaved to create ionic active sites which can absorb moisture to become silanol. This undesirably brings about a viscosity rise and cure of the silicone product, detracting from shelf stability.